The Eastern Pacific’s unending parade of tropical cyclones continues. The latest member of the show is Hurricane Estelle, which got its name Friday night. Joining the party Tropical Storm Agatha started on July 2 have been Category 4 Hurricane Blas, Category 2 Hurricane Celia, Category 3 Hurricane Darby, and soon-to-be Category 1 Hurricane Estelle (Estelle was a high-end tropical storm with 70 mph winds at 11 am EDT Monday.) This puts us well ahead of climatology: the Eastern Pacific usually does not see its fifth named storm until July 22, its fourth hurricane until August 12, and its second major hurricane until August 19. An average season has 15 named storms, 8 hurricanes, and 3 major hurricanes.

Frank and Georgette on the way?
In their 8 am EDT Monday Tropical Weather Outlook, NHC identified two more areas of possible tropical cyclone formation in the Eastern Pacific off the Pacific coast of Mexico. They gave 2-day and 5-day odds of development of 0% and 20% to one area, and 0% and 50% to the other. Both the European and GFS models show the potential for these areas of concern to become Tropical Storm Frank and Tropical Storm Georgette by late this week or early next week–though the models are not as gung-ho about developing these systems as they were for Agatha, Blas, Celia, Darby, and Estelle. The two potential new storms are expected to take a track to the west or west-northwest away from or parallel to the coast Mexico. The July record for named storms forming in the Eastern Pacific is seven, set in 1985, according to NHC hurricane scientist Eric Blake. If we get a Tropical Storm Georgette this year, that would tie the July record.

The Atlantic remains quiet–but beware of this year’s ocean heat content!
As is usually the case when the Eastern Pacific is active, the Atlantic is quiet. This inverse correlation in activity occurs because the conditions over the Eastern Pacific driving this July’s bounteous activity–surface low pressure and rising air–creates a compensating area of sinking air over the tropical Atlantic. This sinking air creates surface high pressure and dry weather–the antithesis of conditions needed for tropical cyclone formation. There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days. Don’t expect to see much activity in the Atlantic until the Eastern Pacific’s burst of activity slows down. When we finally do get the surface low pressure, rising air, low wind shear, plentiful low to mid-level moisture and an African tropical wave needed to spawn an Atlantic hurricane, watch out. Record to near-record levels of heat energy are in the Atlantic in the Caribbean, Gulf of Mexico and waters surrounding the Bahamas (Figure 2), exceeding even the heat energy that was available during the notorious Hurricane Season of 2005. This year’s high levels of ocean heat content in the Atlantic increases the odds of dangerous rapidly-intensifying major hurricanes if the other conditions needed for intensification are present.

Figure 2. Total oceanic heat content (called the Tropical Cyclone Heat Potential, or TCHP) in kilojoules per square centimeter (kJ/cm^2), for July 15 for the years 2005 – 2016. TCHP was at near-record or record values over much of the Caribbean, Gulf of Mexico, and waters surrounding the Bahamas in July 2016. TCHP in excess of 90 kJ/cm^2 (orange colors) is commonly associated with rapid intensification of hurricanes. Image credit: NOAA/AOML.

Joaquin Close to Category 5 Strength; Rains Inundate Carolinas

By: Bob Henson , 6:01 PM GMT on October 03, 2015

There is plenty of life left in Hurricane Joaquin as it moves away from the Bahamas. An Air Force Hurricane Hunter aircraft detected winds around noon EDT Saturday of 144 knots at the 700-millibar level, with stepped-frequency microwave radiometer (SFMR) data showing estimated surface winds of 138 knots (159 mph). The National Hurricane Center upgraded Joaquin’s strength to top sustained winds of 155 mph in a special advisory at noon EDT Saturday, up from 130 mph in the advisory issued just an hour earlier. This immediately pushed Joaquin from the bottom to the top end of the Category 4 scale. A central pressure of 933 millibars was reported, although a radiosonde deployed in the eye of Joaquin failed, so there is some uncertainty around this estimate. Another Hurricane Hunter aircraft was en route to Joaquin as of early Saturday afternoon. Joaquin’s eye has warmed and cleared over the last few hours, reflecting the rapid restrengthening, although infrared satellite imagery shows that its core of strongest thunderstorms has become smaller and less intense.

Figure 1. Latest satellite image of Hurricane Joaquin.

Joaquin’s burst of strength is especially remarkable given that a strong El Niño is under way (El Niño tends to suppress Atlantic hurricane activity by enhancing wind shear). The last Atlantic storm with sustained winds this strong was Hurricane Igor, in 2010, which peaked at 155 mph. The Atlantic’s last Category 5 was Hurricane Felix, in 2007, with winds topping out at 160 mph. The last El Niño season that managed to produce a Category 5 was 2004, when Ivan formed. However, the El Niño event of 2004-05 was relatively weak, with autumn Niño3.4 anomalies of only around +0.7°C compared to the current value of more than +2.0°C.

Joaquin is also in an area where very few Category 5 track segments have been reported since reliable records began in 1950 (see Figure 2). Record-warm waters in this part of the Northwest Atlantic are likely playing a major role in Joaquin’s unusual strength. Joaquin was designated as a tropical depression on Sunday night, September 27, at latitude 27.5°N. This makes Joaquin one of the strongest Atlantic hurricanes on record to have begun its life as a tropical cyclone at such a high latitude. In fact, Joaquin’s latest location (26.4°N. 70.9°W) is still south of its origin point.

Fortunately for the United States, Joaquin is hustling into the open Atlantic, now moving northeast at 16 mph. Track models are fairly consistent in keeping Joaquin west of Bermuda, but with only a small margin for error. Bermuda is now under a hurricane watch and tropical storm warning; at a minimum, the island can expect high surf, strong winds, and a few squalls from outer-edge rainbands, especially as Joaquin makes its closest approach on Monday.

Figure 2. In this map of all Category 5 hurricanes reported in the Atlantic since 1950, bright purple indicates the segments where Category 5 strength was analyzed. Image credit: The Weather Channel, courtesy Jon Erdman.

Figure 3. Satellite image Hurricane Joaquin taken at noon EDT October 3, 2015. At the time, the hurricane was just below Category 5 strength with top winds of 155 mph. A band of very heavy rain can also been seen feeding into South Carolina, to the northwest of the hurricane. Image credit: NASA/GSFC.

Severe flooding likely in South Carolina Saturday and Sunday
As expected, a band of torrential rain has materialized over South Carolina, paving the way for an especially dangerous situation from Saturday afternoon into Sunday. As of midday Saturday, the heaviest rain extended from the south half of the South Carolina coastline northwest across the state to the hilly Uplands region. The swath of intense rain will pivot very slowly in a counterclockwise direction, gradually translating southward over the higher terrain but moving very little near the coastline. This will put the area around Charleston at particular risk of severe flash flooding from Saturday afternoon into Sunday. CoCoRaHS maps show widespread rain totals of 4” – 8” in the Charleston area from 7:00 am EDT Friday to 7:00 am Saturday.

Figure 5. Predicted 15-hour rainfall totals from the HRRR model for the period from 10:00 am Saturday, October 3, to 1:00 am Sunday, October 4. Image credit: NWS/NCEP.

The Charleston area has a reasonable chance of beating the all-time three-day rainfall records below, possibly in just a 24-hour period!

Forecasters are particularly concerned that high-tide cycles in Charleston may coincide with periods of torrential rain, which could produce extreme flash flooding in the city in short order. The Saturday afternoon high tide of 8.2 feet was the highest to occur since Hurricane Hugo in 1989. The next tides will occur in Charleston at 1:34 am and 2:03 pm on Sunday.

Surrounding states are also experiencing heavy rain and flood threats. Mudslides and landslides are possible in the higher terrain of Georgia, South Carolina, and North Carolina. A strengthening of the onshore flow that has persisted for several days over the mid-Atlantic will again raise the risk of significant tidal flooding from Virginia to New Jersey, especially in the Hampton Roads area of Virginia.

Tropical Storm Oho may threaten Hawaii
The hyperactive Central Pacific broke its record–again–for the most number of named storms in a single season with the christening of Tropical Storm Oho on Saturday. According to NHC’s Eric Blake, Oho is the eighth tropical storm to form in the Central Pacific this year, doubling the previous record of just four. Oho is now located roughly 500 miles south-southeast of Honolulu. The steering patterns that will drive Oho are ill-defined and still evolving, which complicates the track forecast. The Central Pacific Hurricane Center currently projects Oho to arc northwest over the next couple of days, then move more briskly toward the east and northeast on a path that would keep it a couple hundred miles south of Hawaii’s Big Island early next week. There is plenty of room for this forecast to evolve, though. Oho has the chance to become a powerful hurricane, thanks to the weak upper-level flow as well as record-warm waters that have fueled so many other tropical cyclones in the Central Pacific this year. The SHIPS rapid intensification index gives Oho a good chance of rapidly strengthening from Saturday into Sunday. Oho now has top sustained winds of just 40 mph, but most dynamical and statistical models are making Oho a hurricane by Monday, and several bring it to Category 2 status by Thursday.

Elsewhere in the tropics
An array of other systems peppered the Northern Hemisphere tropics on Saturday. In the Central Atlantic, Invest 90L is looking less robust, with NHC now giving it only a 40% chance of development in the next 2 to 5 days. A late-blooming Cape Verde wave between 30°W and 35°W poses little threat over at least the next several days, and strong wind shear at low latitudes will probably cap any later development.

In the Northeast Pacific, Invest 94E is slowly organizing more than 1000 miles southwest of Baja California. NHC gives 94E a 30% chance of developing into a tropical cyclone by Monday and a 50% chance by Thursday. Closer to Mexico, the remants of Tropical Storm Marty could produce heavy rainfall as they move inland on Sunday into Monday. Some moisture from ex-Marty may get entrained into an upper-level storm taking shape early next week in the Southwest U.S., possibly delivering strong thunderstorms to the Arizona deserts on Monday.

In the Central Pacific, still another system–Tropical Depression 8C, the 13th tropical cyclone to form or pass through the Central Pacific this year–formed on Saturday morning about 1100 miles southwest of Honolulu. Moderate southerly shear should keep 8C from developing beyond minimal tropical-storm strength for at least the next couple of days as it pushes westward.

In the Northwest Pacific, Typhoon Mujigae may strengthen slightly over the next 24 hours before it moves into the coast of extreme southern China, southwest of Hong Kong. To the east, Tropical Storm Choi-Wan will slowly gather steam and may become a minimal typhoon early next week before an expected recurvature just east of Japan by midweek.

Hurricane Warnings for the Bahamas From Joaquin; Threat to U.S. East Coast Grows

By: Jeff Masters and Bob Henson , 3:38 PM GMT on September 30, 2015

Joaquin is now a hurricane, and Hurricane Warnings are up for the Central Bahama Islands as the slowly intensifying storm moves southwest at 6 mph. An Air Force hurricane hunter aircraft made two penetrations of Joaquin’s center on Wednesday morning, and found top surface winds of 80 mph, a central pressure of 971 mb, and a huge 54-mile diameter eye with a fully closed eyewall. Joaquin continues to battle high wind shear of 20 knots due to strong upper-level winds out of the north-northwest, but this wind shear had fallen by about 5 knots since Tuesday morning. Water vapor satellite loops show that a large area of dry air lay to the northwest of Joaquin, and the strong wind shear was driving this dry air into Joaquin’s core, keeping intensification slow. Visible and infrared satellite loops show that Joaquin has developed a large Central Dense Overcast (CDO) of high cirrus clouds over the center, characteristic of intensifying storms, and the hurricane’s large eye was beginning to be apparent. Upper level winds analyses from the University of Wisconsin show that the hurricane has developed an impressive upper-level outflow channel to the southeast, which is supporting the intensification process. Ocean temperatures in the region are near 30°C (86°F)–the warmest seen there since record keeping began in 1880.

Figure 1. Latest satellite image of Joaquin.

The U.S. outlook for Joaquin
A hurricane watch could be required for portions of the U.S. East Coast as early as Thursday night. The forecast for Joaquin is very complex, and the confidence in both the intensity and track forecast for the storm is low. Joaquin is trapped to the south of a high pressure system whose clockwise flow will push the cyclone very slowly to the southwest or west-southwest at about 3 – 6 mph. As the storm progresses to the southwest, the strong upper-level winds out of the north currently bringing high wind shear of 20 knots will gradually decrease, continuing to allow Joaquin to strengthen. The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear over Joaquin would fall to the moderate range, 10 – 20 knots, on Thursday and Friday. These conditions should allow Joaquin to intensify to a Category 2 hurricane by Thursday. As Joaquin progresses to the west, the storm will also increasingly “feel” the steering influence of a strong upper-level trough of low pressure situated over the Eastern United States on Friday, and begin to turn north. These winds may also open up another upper-level outflow channel to the northwest of Joaquin on Friday, potentially allowing the storm to intensify to Category 3 strength. However, as Joaquin gets closer to this trough, its winds will bring high wind shear of 20+ knots, likely halting the intensification process and causing weakening by Sunday.

Figure 2. Our two top models for forecasting hurricane tracks, both run at 8 pm EDT Tuesday September 29, 2015 (00Z Wednesday) , came up with two very different solutions for the path of Joaquin. The GFS model showed Joaquin making landfall in Virginia, while the European model took the storm to the northeast out to sea without hitting the U.S. Image credit: wundermap with the “Model Data” layer turned on.

Figure 3. The ensemble runs of our two top models for forecasting hurricane tracks, both run at 8 pm EDT Tuesday September 29, 2015 (00Z Wednesday). The 50 members of the European model ensemble (top) and the 20 members of the GFS model ensemble (bottom) both had numerous model runs that took Joaquin into U.S. East Coast, and ones that missed the U.S. coast entirely. Ensemble runs take the operational version of the model and run it at lower resolution with slightly different initial conditions, to generate an “ensemble” of possible forecasts. The operation high-resolution (and presumably best-guess) forecast for the models is shown in red. The European model ensemble had four members that tracked the movement of Joaquin exceptionally well during the previous 12 hours; three of those four members had tracks for Joaquin that missed the U.S., and one that hit the coast near New York City. Image taken from a custom software package used by TWC.

The big trend from the 00Z Wednesday (8 pm EDT Tuesday) suite of computer model guidance was a marked convergence toward a landfall in the vicinity of North Carolina or Virginia. The 00Z HWRF and GFDL models were joined by the 00Z GFS in hooking Joaquin toward the northwest on Friday and accelerating the hurricane into the coast between Cape Hatteras, NC, and the Delmarva Peninsula as a significant hurricane on Saturday/Sunday. The high-resolution HWRF and GFDL output showed central pressures in the 940-950 mb range at landfall, with wind speeds on par with a Category 2 hurricane. The 00Z UKMET solution angled more north-northwestward, with Joaquin arriving near the southern end of the Delmarva and scraping up the coast into eastern New Jersey and New York. Among the major dynamical models, only the European (ECMWF) model remained adamant that Joaquin would head to sea well before reaching the southeast U.S., although its 00Z track was a bit west of previous runs. The leftward hook prominently featured in the other models is being driven by the increasingly negative tilt (NW-to-SE) to the upper trough deepening over the eastern U.S. late this week. The models are projecting that this trough would pull in Joaquin on its northeast side, in much the same way that a strong upper-level low did with Hurricane/Superstorm Sandy in 2012. However, in Joaquin’s case, the process would unfold a couple of hundred miles to the south. The ECMWF run shows a very similar upper-level pattern to the other models, but the timing of the trough’s interaction with Joaquin and with Invest 90L is such that the hurricane is shunted to sea instead of being tucked into the northeast side of the trough. 90L was centered at 8 am EDT Wednesday about 1000 miles east-northeast of Joaquin. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 90L 2-day and 5-day odds of development of 40% and 70%, respectively.

Figure 4. Model track guidance initialized at 12Z Wednesday (8 am EDT) shows a continued clustering of model solutions toward North Carolina and the mid-Atlantic. This early-track guidance uses 12Z data on Joaquin to update the previous model runs from 06Z. This map does not include the ECMWF model, whose 00Z operational run took Joaquin out to sea. Image credit: Levi Cowan, tropicaltidbits.com.

Back in 2012, the ECMWF model caught on to the leftward hook of Sandy’s track several days before other models. The ECMWF’s high overall skill means we cannot entirely discount its out-to-sea forecast for Joaquin just yet. At the same time, the strong consistency among other leading models in projecting a NC/mid-Atlantic landfall cannot be ignored. We can gain more perspective on this scenario by looking at the ECMWF and GFS ensemble output from 00Z Wednesday. In each ensemble, the model is run a number of times for the same situation, but with the starting conditions varied slightly to represent the uncertainty in our starting-point observations of the atmosphere. The ECMWF and GFS ensembles from 00Z Wednesday are much more similar in flavor than you might expect from looking at their single operational runs. Both models have a majority of ensemble members heading for North Carolina and the mid-Atlantic, with a few outliers heading to sea.

If Wednesday’s 12Z (8 am EDT) models continue to zero in on a NC/VA landfall, and especially if the ECMWF comes more fully around, then this solution will become a more high-confidence forecast. The NHC has been nudging its “cone of uncertainty” toward the left, still splitting the difference between the ECMWF and other solutions while acknowledging the westward trend. The entire U.S. coast from the Outer Banks of NC to southern New England was located in the 5-day cone issued at 11 am EDT Wednesday and valid at 2 am EDT Monday. Even if NHC moves more fully toward the NC/mid-Atlantic scenario, we can still expect to see a large swath of coastline remaining in the “cone” as we get closer to Joaquin’s eventual landfall.

Potential impacts from Joaquin
Apart from the remaining uncertainty about a U.S. landfall, Joaquin is now poised to bring hurricane-force conditions into or very close to the southeastern Bahamas. WIth luck, these islands will remain on the weaker left-hand side of Joaquin. If the hurricane makes a sharp turn to the north on Friday as predicted, the effects should be considerably less on the northwestern Bahamas.

It is relatively rare for a hurricane to make a Sandy-like left hook into the U.S. East Coast. Such a track was unprecedented for New Jersey in hurricane annals, and even in the NC/VA area, it is uncommon enough that the likely effects would be both unusual and high-impact. The closest analogue from recent years is 2003’s Hurricane Isabel. After a much longer life as a Cape Verde system and a Category 4 hurricane from the Central Atlantic (briefly a Category 5), Isabel angled sharply northwestward and made landfall on North Carolina’s Outer Banks as a strong Category 2 hurricane. Isabel then continued on a fairly direct track to western Pennsylvania as it weakened. Isabel’s trajectory brought huge surf to the coast from North Carolina to New Jersey, with a major storm surge pushing into the Chesapeake Bay and nearby waterways, plus widespread impacts from high wind and heavy rain. Joaquin is not as large or long-lived a storm as Isabel, but if it moved slightly to the north of Isabel’s path, its track could be even more favorable for a Chesapeake surge. Hurricane-force winds would be another factor to contend with, especially just north of Joaquin’s track during and just after landfall. Storm surge expert Dr. Hal Needham of LSU has a detailed look at the potential for storm surge from Joaquin along the U.S. East Coast in his Wednesday morning blog post, Widespread Storm Surge Event to Impact U.S. Atlantic Coast.

One very worrisome aspect of Joaquin is the torrential rains that it could bring from the Carolinas to the Northeast and perhaps even New England. Heavy rains and scattered flash flooding have already occurred in parts of these areas over the last 24 hours, as a preexisting front is overtopped by near-record amounts of water vapor streaming over the region ahead of the trough that will help steer Joaquin. The hurricane itself, arriving after several days of antecedent rainfall, has the potential to produce truly historic rainfall totals. This morning’s 7-day outlook from the NOAA Weather Prediction Center, which goes with the NC/mid-Atlantic scenario, shows widespread 5-10″ amounts from North Carolina to southern New England. Model output suggests that localized 7-day totals of 10-20″ or more are not out of the question, depending on Joaquin’s exact track. We’ll have more on the ongoing and potential flood risk in our afternoon post.

Extreme weather events caused $110 billion in damages in 2012 making it the second costliest year on record, according to NOAA. Superstorm Sandy caused the brunt of that cost, resulting in approximately $65 billion dollars in damage.

This year, as Atlantic hurricane season ramps up, NOAA is implementing a few big changes. Below are the ones that could affect you as we enter the peak of the season:

The National Weather Service (NWS) underwent a major upgrade to their supercomputers this year that more than doubled their computing capacity. According to the agency, this has made them twice as fast in processing computer models, and will allow them to provide more accurate forecasts further out in time. With improved forecasts anticipated, the NWS has decreased the size of their forecast cone for tropical systems, resulting in a more specific track map for storms.

2. A New Symbol Will Represent Tropical Depressions

The National Hurricane Center (NHC) has done away with the old symbol that indicates a tropical depression and replaced it with a hollow circle.

3. Hurricane and Tropical Storms Watch and Warnings Are Redefined

After Superstorm Sandy, the NWS made the decision to modify the hurricane and tropical storm watch and warning definitions to allow them to be used after a system has become post-tropical. Additionally, they now have the option to continue issuing advisories after a system has become post-tropical, in the case that it continues to pose a significant threat to life and property. With this change, the public can anticipate seeing more advisories, continuing awareness of potential threats long after the term “hurricane” or “tropical storm” is discontinued.

4. Tropical Weather Outlooks Will Now Look Forward Five Days

As of Aug. 1, the NHC will now be providing tropical outlooks that look five days forward. A tropical outlook indicates where disturbed weather is located and where there is potential for tropical development. Previously, the outlook only extended 48 hours out. This is the first change in the outlook time period in several decades, according to the NHC.

A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week’s dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.

Figure 1. A massive dust storm moves off the coast of Africa in this MODIS image taken at 1:40 UTC July 30, 2013. Image credit: NASA.

How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion–a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this “inversion” acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm’s updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as “condensation nuclei”–centers around which raindrops can form and grow–the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)

Dust in Africa’s Sahel region and Atlantic hurricane activity
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° – 22° N) and the northwestern Sahel (15° – 18° N) (Figure 3.) The dust from the Southwest Sahara stays relatively constant from year to year, but the dust from the Northwest Sahel varies significantly, so understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2012 (Figure 3), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic this fall, increasing the odds of a busy 2013 hurricane season.

Figure 3. Rainfall over the Northwest Sahel region of Africa was about 200% of average during the 2012 rainy season. The heavy rains promoted vigorous vegetation growth in 2013, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

Dr. Amato Evan published a study in Science magazine March 2009 showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Tropical Storm Chantal is strengthening as it speeds west-northwestwards at 26 mph through the Lesser Antilles Islands. At 6:22 am AST, St. Lucia recorded a wind gust of 54 mph. Sustained winds of 38 mph, gusting to 52 mph, were observed at Martinique at 10 am AST. An Air Force hurricane hunter aircraft is in the storm, and measured top winds at their 1,000′ flight level of 80 mph at 8:41 am AST, about 20 miles north of the center. Top winds seen by the aircraft’s SFMR instrument were about 60 mph, and it is likely that NHC will bump up Chantal’s top winds to at least 60 mph in their 11 am advisory. Barbados Radar shows a large area of heavy rain that has organized moderately well into low-level spiral bands affecting much of the central Lesser Antilles Islands. Chantal is not very impressive on satellite loops, though, with only a modest amount of heavy thunderstorms that are not well-organized. Only a small amount of upper-level outflow is visible. Chantal is fighting dry air associated with the Saharan Air Layer (SAL), as seen on water vapor satellite loops. Moderate wind shear of 15 – 20 knots is driving dry air into the storm. Ocean temperatures are fairly warm, at 28°C.

Figure 1. Latest satellite image of Chantal.

Figure 2. Barbados weather radar image of Chantal taken at 9:14 am AST on Tuesday, July 9, 2013. Chantal’s center was located between St. Lucia and Martinique. Image credit: Barbados Met Service.

Forecast for Chantal
Chantal will likely continue to intensify before hitting Hispaniola on Wednesday afternoon. In their 5 am EDT wind probability forecast, NHC gave Chantal a 23% chance of becoming a hurricane before hitting Hispaniola. Working against intensification will be the fast forward speed of the storm–tropical storms moving faster than 20 mph in the deep tropics usually have trouble intensifying. In addition, the Eastern Caribbean is an area where the trade winds accelerate, helping drive sinking air that discourages tropical storm intensification. Dry air will also slow down the intensification process. Interaction with the high mountains of Hispaniola and high wind shear may be able to destroy Chantal by Thursday. The 8 am EDT Tuesday wind shear forecast from the SHIPS model calls for shear to rise to the high range, 20 – 30 knots, as the storm approaches and crosses Hispaniola on Wednesday and Thursday. On Friday and Saturday, when Chantal is expected to be in the Bahamas, lower moderate wind shear of 15 – 20 knots should allow for re-intensification of the storm–if it survives interaction with the high mountains of Hispaniola, Puerto Rico, and Eastern Cuba. The latest 06Z run of the GFS model dissipates Chantal as it crosses Hispaniola, though the 00Z run done 6 hours earlier had the storm surviving. Chantal has the potential to cause big problems for Haiti, which is highly vulnerable to flash flooding due to the lack of vegetation on the deforested mountains. However, there is a lot of dry air to the west of Chantal, which may act to keep rainfall totals in Haiti down to a manageable 2 – 4″. Over 300,000 people are still homeless and living in makeshift tent camps in Haiti, three years after the great 2010 earthquake.

Chantal’s fast west-northwest forward speed of 26 mph will slow to 20 mph by Wednesday afternoon and then 10 mph by Thursday afternoon, as the storm “feels” the presence of a trough of low pressure over the U.S. East Coast. This trough will steer Chantal to the northwest and then north-northwest across Hispaniola and into the Bahamas. The trough of low pressure pulling Chantal northwards is expected to lift out the the northeast over the weekend, leaving Chantal behind off the coast of Florida. High pressure will likely build in, potentially forcing an intensifying Chantal westwards into the Florida or Southeast U.S. coast, with a possible Monday landfall.

It was yet another active day for tornadoes, flooding, and severe thunderstorms in the Midwest on Thursday, with NOAA’s Storm Prediction Center (SPC) logging 16 preliminary tornado reports. Twisters touched down in Oklahoma, Arkansas, and Illinois. The tornadoes missed heavily populated areas for the most part, but seven people were injured in Arkansas in two separate tornadoes, and two other people were hurt by lightning. The severe weather forced organizers of the outdoor Wakarusa Music Festival north of Ozark, Arkansas to delay the start of the festival. The band “Widespread Panic” was one of the groups scheduled to perform, leading to an Associated Press headline from yesterday titled, “Nine hurt in Arkansas storm; Widespread Panic delayed.” Heavy rains from this week’s thunderstorms have pushed the Mississippi River to major flood stage at most places from Burlington Iowa to Quincy Illinois, and the river is expected to crest near major flood stage at St. Louis early next week. In Iowa, the Cedar River at Cedar Falls, the Iowa River at Marengo, and the Skunk River near Sigourney and at Augusta are also in major flood. The latest forecast from NOAA’s Storm Prediction Center calls for a “Moderate Risk” of severe weather today (Friday) over much of Oklahoma and Southwest Missouri, including Oklahoma City and Tulsa in Oklahoma, and Joplin, Missouri, with the potential for several strong EF-2 and EF-3 tornadoes.

Figure 1. Lightning strike from a severe thunderstorm near Guthrie, Oklahoma on May 30, 2013, as photographed by KFOR-TV. (AP Photo/KFOR-TV)

Figure 2. Severe weather outlook for Thursday, May 30, calls for a “Moderate Risk” of severe weather over much of Oklahoma and Southwest Missouri, including Oklahoma City and Tulsa in Oklahoma, and Joplin, Missouri. You can follow today’s severe weather outbreak from our Severe Weather page.

A mostly quiet year for violent tornadoes
After a very quiet March, April, and first half of May, the U.S. tornado season has become very active during the last half of May, and is beginning to catch up to normal. TWC’s tornado expert Dr. Greg Forbes has a preliminary count of 181 tornadoes for the month of May, through May 29, which is 35% below the 10-year average of 279 through May 29th. May 2012 had only 121 tornadoes. The 2013 tornado tally has risen significantly in the last half of May, due to 7 of the last 15 days having above-average numbers of tornadoes. Fortunately, we are well below-average for strong and violent EF-3, EF-4, and EF-5 tornadoes so far in 2013. According to NOAA, the U.S. has averaged 43 EF-3 or stronger tornadoes per year during the period 1954 – 2012. With tornado season nearly half over, only twelve EF-3 and EF-4 tornadoes have been recorded so far in 2013. An average year should have had at least twenty of these tornadoes by this point in the year. Here are the twelve EF-3 and stronger tornadoes so far in 2013, as detailed in Wikipedia’s excellent Tornadoes of 2013 page:

Figure 3. The annual number of EF-3 and stronger tornadoes, 1954 – 2012. The greatest number of these dangerous tornadoes was 131 in 1974, the year of the notorious “Super Outbreak.” The minimum was just 15, set in 1987. The average is 43 per year. Image credit: NOAA.

Video 1. Impressive 2-minute timelapse of the Bennington, Kansas wedge tornado of May 28, 2013, as filmed by the Aussie Storm Chasers. As discussed in an excellent blog post by Jason Samenow of the Capital Weather Gang, the violent EF-4 tornado stood still for nearly an hour, and had wind gusts as high as 264 mph at an altitude of 300 feet measured by Doppler on Wheels.

Remains of Hurricane Barbara bringing heavy rains to Mexico
Hurricane Barbara died on Thursday as it attempted to cross Mexico’s Isthmus of Tehuantepec into the southernmost Gulf of Mexico. While there is no low level circulation apparent on satellite loops this Friday morning, there is a bit of spin at middle levels of the atmosphere, and the remains of Barbara are kicking up some heavy thunderstorm activity over the southernmost Gulf of Mexico and adjacent land areas of Mexico. Wind shear is a high 20 knots in the region, and the area of disturbed weather is quite small, so I don’t expect any development to occur over the next few days. Wind shear is predicted to remain high over the Gulf of Mexico for the next six days, and none of the reliable computers models is calling for tropical cyclone development in the Atlantic during that period. Late next week, wind shear is predicted to drop, and there is a better chance for tropical cyclone development in the Gulf of Mexico or Western Caribbean. Both the GFS and ECMWF models suggest that a strong tropical disturbance with heavy rains may affect Mexico’s Yucatan Peninsula, Western Cuba, and the Southwest Florida by Friday next week.

Figure 4. Remains of Hurricane Barbara in the southernmost Gulf of Mexico as seen by MODIS at 12:05 pm EDT Thursday, May 30, 2013. Barbara had just been declared dead one hour prior to this photo. Image credit: NASA.

Saturday, June 1, is the first day of hurricane season, and I’ll post a quick look at what we might expect to see in June.

NOAA forecasts an above-normal and possibly very active Atlantic hurricane season in 2013, in their May 23 outlook. They give a 70% chance of an above-normal season, a 25% chance of an near-normal season, and 5% chance of a below-normal season. They predict a 70% chance that there will be 13 – 20 named storms, 7 – 11 hurricanes, and 3 – 6 major hurricanes, with an Accumulated Cyclone Energy (ACE) 120% – 205% of the median. If we take the midpoint of these numbers, NOAA is calling for 16.5 named storms, 9 hurricanes, 4.5 major hurricanes, and an ACE index 162% of normal. This is well above the 1981 – 2010 average of 12 named storms, 6 hurricanes, and 3 major hurricanes. Hurricane seasons during the active hurricane period 1995 – 2012 have averaged 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index 151% of the median. Only five seasons since the active hurricane period that began in 1995 have not been above normal–including four El Niño years (1997, 2002, 2006, and 2009), and the neutral 2007 season.

Figure 1.Hurricane Michael as seen by NASA’s Aqua satellite at 12:20 pm EDT Thursday September 6, 2012. At the time, Michael was a major Category 3 hurricane with 115 mph winds. Hurricane Sandy was the only other major Atlantic hurricane of 2012. Image credit: NASA.

The forecasters cited the following main factors that will influence the coming season:

1) Above-average sea surface temperatures (SSTs) are expected in the hurricane Main Development Region (MDR), from the Caribbean to the coast of Africa between between 10°N and 20°N. SSTs in the MDR during April were 0.4°C above average, and were 0.33°C above the oceans in the remainder of the global tropics. Long-range seasonal computer model forecasts predict a continuation of above-average SSTs in the MDR during much of hurricane season.

2) We are in an active period of hurricane activity that began in 1995, thanks to a natural decades-long cycle in hurricane activity called the Atlantic Multi-decadal Oscillation (AMO).

3) No El Niño event is expected this year. El Niño events tend to suppress Atlantic hurricane activity. Neutral conditions have been present since last summer, and are predicted to remain neutral through hurricane season by most of the El Niño computer forecast models.

NOAA said, “This combination of climate factors historically produces above-normal Atlantic hurricane seasons. The 2013 hurricane season could see activity comparable to some of the very active seasons since 1995.” NOAA is increasingly using output from ultra-long range runs of the computer forecast models we rely on to make day-to-day weather forecasts, for their seasonal hurricane forecasts. These models include the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) model CM2.1, the European Centre for Medium Range Weather Forecasting (ECMWF) model, the United Kingdom Meteorology (UKMET) office model, and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble.

How accurate are NOAA’s seasonal hurricane forecasts?A talk presented by NHC’s Eric Blake at the 2010 29th Annual AMS Conference on Hurricanes and Tropical Meteorology studied the accuracy of NOAA’s late May seasonal Atlantic hurricane forecasts, using the mid-point of the range given for the number of named storms, hurricanes, intense hurricanes, and ACE index. Over the past twelve years, a forecast made using climatology was in error, on average, by 3.6 named storms, 2.5 hurricanes, and 1.7 intense hurricanes. NOAA’s May forecast was not significantly better than climatology for these quantities, with average errors of 3.5 named storms, 2.3 hurricanes, and 1.4 intense hurricanes. Only NOAA’s May ACE forecast was significantly better than climatology, averaging 58 ACE units off, compared to the 74 for climatology. Using another way to measure skill, the Mean Squared Error, May NOAA forecasts for named storms, hurricanes, and intense hurricanes had a skill of between 5% and 21% over a climatology forecast. Not surprisingly, NOAA’s August forecasts were much better than the May forecasts, and did significantly better than a climatology forecast.

Figure 3. Forecast skill of the TSR, NOAA (National Oceanic and Atmospheric Administration) and CSU (Colorado State University) for the number of hurricanes in the Atlantic during 2003-2012, as a function of lead time. Forecast precision is assessed using the Mean Square Skill Score (MSSS) which is the percentage improvement in mean square error over a climatology forecast (six hurricanes in a given year.) Positive skill indicates that the model performs better than climatology, while a negative skill indicates that it performs worse than climatology. Two different climatologies are used: a fixed 50-year (1950-1999) climatology, and a running prior 10-year climate norm. NOAA does not release seasonal outlooks before late May, and CSU stopped providing quantitative extended-range December hurricane outlooks in 2011. Skill climbs as the hurricane season approaches, with modest skill levels by early June, and good skill levels by early August. Image credit: Tropical Storm Risk, Inc (TSR).

TSR predicts an active hurricane season: 15.3 named storms
The May 24 forecast for the 2013 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 15.3 named storms, 7.5 hurricanes, 3.4 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 130. The long-term averages for the past 63 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 103. TSR rates their skill level as modest for these late May forecasts–11% – 25% higher than a “no-skill” forecast made using climatology. TSR predicts a 63% chance that U.S. land falling activity will be above average, a 21% chance it will be near average, and a 16% chance it will be below average. They project that 4.4 named storms will hit the U.S., with 2 of these being hurricanes. The averages from the 1950-2012 climatology are 3.1 named storms and 1.4 hurricanes. They rate their skill at making these late May forecasts for U.S. landfalls just 8% – 12% higher than a “no-skill” forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.5 named storms, 0.6 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR’s two predictors for their statistical model are the forecast July – September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August – September 2013 sea surface temperatures in the tropical North Atlantic. Their model is calling for warmer than average SSTs and slower than average trade winds during these periods, and both of these factors should act to increase hurricane and tropical storm activity.

UKMET office predicts a slightly above normal Atlantic hurricane season: 14 named storms
The UKMET office forecast for the 2013 Atlantic hurricane season, issued May 13, calls for slightly above normal activity, with 14 named storms, 9 hurricanes, and an ACE index of 130. In contrast to the statistical models relied upon by CSU, TSR, and NOAA, the UKMET forecast is done strictly using two dynamical global seasonal prediction systems: the Met Office GloSea5 system and ECMWF system 4. In 2012, the Met Office forecast was for 10 tropical storms and an ACE index of 90. The actual numbers were 19 named storms and an ACE index of 123.

WSI predicts an active hurricane season: 16 named storms
The April 8 forecast from the private weather firm WSI (part of The Weather Company, along with The Weather Channel, Weather Central, and The Weather Underground), is calling for an active season with 16 named storms, 9 hurricanes, and 5 intense hurricanes.

Penn State predicts an active hurricane season: 16 named storms
The May 11 forecast made using a statistical model by Penn State’s Michael Mann and alumnus Michael Kozar is calling for an active Atlantic hurricane season with 16 named storms, plus or minus 4 storms. Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistic model assumes that in 2013 the May 0.87°C above average temperatures in the MDR will persist throughout hurricane season, the El Niño phase will be neutral, and the North Atlantic Oscillation (NAO) will be near average.

The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done pretty well, except for in 2012, when an expected El Niño did not materialize:

The wunderground community predicts an active hurricane season: 17 named storms
Over 100 members of the wunderground community have submitted their seasonal hurricane forecasts, which are compiled on trHUrrIXC5MMX’s blog. The April 28 version of this list called for an average of 17 named storms, 8 hurricanes, and 4 intense hurricanes in the Atlantic. This list will be updated by June 3, so get your forecasts in by then! As usual, I am abstaining from making a hurricane season forecast. I figure there’s no sense making a forecast that will be wrong nearly half the time; I prefer to stick to higher-probability forecasts.

NOAA predicts a below-average Eastern Pacific hurricane season: 13.5 named stormsNOAA’s pre-season prediction for the Eastern Pacific hurricane season, issued on May 23, calls for a below-average season, with 11 – 16 named storms, 5 – 8 hurricanes, 1 – 4 major hurricanes, and an ACE index 60% – 105% of the median. The mid-point of these ranges gives us a forecast for 13.5 named storms, 6.5 hurricanes, and 2.5 major hurricanes, with an ACE index 82% of average. The 1981 – 2010 averages for the Eastern Pacific hurricane season are 15 named storms, 8 hurricanes, and 4 major hurricanes. So far in 2013, there has already been one named storm. On average, the 2nd storm of the year doesn’t form until June 25.

NOAA predicts a below-average Central Pacific hurricane season: 2 tropical cyclonesNOAA’s pre-season prediction for the Central Pacific hurricane season, issued on May 22, calls for a below-average season, with 1 – 3 tropical cyclones. An average season has 4 – 5 tropical cyclones, which include tropical depressions, tropical storms, and hurricanes. Hawaii is the primary land area affected by Central Pacific tropical cyclones.

The week ahead: 91E, and a heavy rainfall threat to Mexico
We’re already behind last year’s pace for named storms in both the Atlantic (where Tropical Storm Alberto formed on May 19, and Tropical Storm Beryl on May 26), and in the Eastern Pacific, where Bud formed on May 21 (the earliest date since record keeping began in 1949 for formation of the season’s second named storm.) The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 – 60 days, is currently located in the Eastern Pacific. The MJO is relatively weak, but is helping boost the chances that Invest 91E in the Eastern Pacific will develop. On Friday, NHC was giving 91E a 20% of developing into a tropical cyclone by Sunday. The 12Z Friday runs of the GFS and ECMWF models were predicting that a weak circulation off the coast of Costa Rica, well east of the separate circulation currently called 91E, could develop into a tropical depression by Tuesday. This system is a threat to spread heavy rains to the coast of Mexico from Acapulco to Guatemala on Tuesday and Wednesday.

In the Atlantic, the models are depicting high wind shear through June 1 over the majority of the regions we typically see May tropical cyclone development–the Caribbean, Gulf of Mexico, and Bahamas. The GFS model is showing a decrease in wind shear over the Western Caribbean after June 1, which would argue for an increased chance of tropical storm development then (though wind shear forecasts more than 7 days in advance are highly unreliable.) The prospects for an early June named storm in the Atlantic are probably above average, though, given that the MJO may be active in the Atlantic during th first week of June.

Tornadoes Were Just the Beginning. This Hurricane Season Is Going to be Stormy

Photo by NASA via Getty ImagesA satellite image of Hurricane Sandy as it approached the East Coast last year

The residents of Moore, Oklahoma are still cleaning up from the EF5 tornado that tore through their town on May 20. 24 people died in the twisters, and thousands of homes and buildings were damaged or destroyed. The total bill may come in at over $2 billion, which would make the Moore tornado the most expensive in American history.

So this may not be the best time, but the Moore tornado almost surely won’t be the last billion-dollar weather the U.S. faces in 2013. On Thursday the National Oceanic and Atmospheric Administration (NOAA) released its annual outlook on the summer Atlantic hurricane season—and it is not good. Technically it will be “active or extremely active,” which is fine if you’re talking about a workout session, and less good if you’re projecting how many potentially devastating tropical storms will hit the U.S. mainland.

Altogether NOAA predicts a 70% likelihood that 13 to 20 named storms—which have winds that sustain at 39 mph or higher—will occur, of which 7 to 11 could become hurricanes (winds higher than 74 mph). Of those three to six may become major hurricanes, which means Category 3 to 5, with winds above 11 mph. That’s all well above the average for an Atlantic hurricane season, which lasts from June 1 to the end of November.

Why will this summer potentially be so stormy? For one, an atmospheric climate pattern, including a strong African monsoon, that’s been ongoing since 1995 will help supercharge the atmosphere for tropical storms. Warmer-than-average water temperatures in the tropical Atlantic and the Caribbean Sea will lead to more of the wet, hot air that provides the fuel for hurricanes. And there is no El Nino—the alternating climate pattern that means unusually warm sea temperatures—which would usually suppress the formation of hurricanes.

It’s important to remember that NOAA is only predicting whether or not hurricanes and tropical storms will develop—not whether they will make landfall like Superstorm Sandy did last fall. Only three of the 19 named storms that formed in the Atlantic last year made enough of an impact on the U.S. to cause any real damage. Most storms form in the Atlantic and never leave. It’s not just the strength of a storm that makes it dangerous—it’s location.

Superstorm Sandy made that clear. By the time storm made landfall on the East Coast, it had actually weakened to the point that it was barely a hurricane at all, though it was an unusually massive and wet storm. Had it spun back out to sea, we never would have remembered its name. Instead, though, Sandy tore through the most populated and expensive property in the U.S., flooding parts of New York City and causing some $65 billion in damage. We can only imagine what kind of destruction it would have caused had Sandy been an even stronger storm.

There’s no way of knowing how many of the storms to come this summer will indeed make landfall, but it stands to reason that the more storms that form, the greater the chance one will eventually end up in our backyard. According to NOAA, billion-dollar disasters are increasing in the U.S. at a rate of about 4.8% a year—there were 11 just last year. That’s mostly a result of economic growth—as the country gets richer, even with inflation, any weather disaster that disrupt the economy will cost more. But climate change is likely playing a role as well—in the case of hurricanes, warming temperatures seem to make storms stronger, and rising sea levels increase the threat of coastal flooding.

In any case, the growing danger from extreme weather just underlines the need to invest in forecasting, preparation and adaptation, as acting NOAA Administrator Kathryn Sullivan said:

With the devastation of Sandy fresh in our minds, and another active season predicted, everyone at NOAA is committed to providing life-saving forecasts in the face of these storms and ensuring that Americans are prepared and ready ahead of time.” said Kathryn Sullivan, Ph.D., NOAA acting administrator. “As we saw first-hand with Sandy, it’s important to remember that tropical storm and hurricane impacts are not limited to the coastline. Strong winds, torrential rain, flooding, and tornadoes often threaten inland areas far from where the storm first makes landfall.

Of course, if you really want to worry, remember that last year NOAA predict that the Atlantic hurricane season would be just a little above normal. It ended up being considerably more active. But there’s one thing we can be sure of—there won’t be another Hurricane Sandy. That name has been retired.

After going twelve months with a record-low tornado death toll of just seven people, last night we received a jolting reminder that tornadoes typically kill a lot more people than that in the U.S. A deadly tornado swept through Granbury, Texas near 8 pm CDT, killing six and injuring up to 100. The weather system that spawned the Granby tornado also unleashed a mile-wide twister that hit Cleburne, about 25 miles southeast of Granbury. Damage was heavy in Cleburne and a state of emergency declared, but only seven minor injuries were reported. A third tornado hit the small town of Millsap, about 40 miles west of Fort Worth, causing roof damage a destroying a barn, but caused no injuries. Preliminary figures indicate that a total ten tornadoes touched down in Texas last night, and NWS damage survey teams are out today to determine the exact total and how strong they were. The National Weather Service out of Fort Worth has issued a preliminary rating of EF-4 to the Granbury tornado, making it the first tornado stronger than EF-0 reported in May 2013. The storms also dumped softball-sized hail up to 4″ in diameter in Mineral Wells, TX.

Video 1. The Granbury, Texas tornado of May 15, 2013.

Thursday’s tornado was the deadliest U.S. tornado in over a year. The last time six people died in a U.S. tornado was on April 14, 2012, during an EF-3 tornado that hit Woodward, Oklahoma. The last Texas tornado that was deadlier occurred on April 24, 2007 in Maverick County, when an EF-3 tornado hit Eagle Pass, Texas, killing seven. Texas has had one other tornado death in 2013, from a twister that hit on February 21, 2013, in Sabine County. The region of Texas hit by last night’s tornadoes has few basements, which may have contributed to the death toll. According to underground member Seattleite, “In this part of Texas basements are very uncommon. The reason is due to the soil, it is basically clay. It contracts and expands with temperature and moisture levels on the order of a foot or more in a typical year. The pressure from this can cause basement walls to cave. They can be built, but it costs at least an extra $20,000+, as they surround the basement with a sand-like barrier to handle the changes in the ground.”

Figure 1. Softball, anyone? One of the 4″ hailstones that fell near Mineral Wells, Texas on May 15, 2013. Image from Patrick Vondra via Twitter.

Is the 2012 – 2013 tornado drought over?
Thanks to the cold spring in the Midwest during 2013, and the 2012 Midwest drought, the 197 EF-1 and stronger tornadoes that occurred during May 2012 – April 2012 was an all-time minimum for any twelve-month period since at least 1954, wrote tornado researcher Harold Brooks at the U.S. Severe Weather Blog (previous minimum: 247 tornadoes from June 1991-May 1992.) The death toll of just seven was also a record low for any twelve-month period since 1950. Amazingly, this tornado drought occurred less than two years after the record maximum: 1050 EF-1 and stronger tornadoes from June 2010 – May 2011. The extraordinary contrast underscores the crazy fluctuations we’ve seen in Northern Hemisphere jet stream patterns during the past three years. Call it “Weather Whiplash” of the tornado variety. A blog post by meteorologist Patrick Marsh of NOAA’s Storm Prediction Center argues that the record 12-month tornado maximum of 1050 EF-1 and stronger tornadoes from June 2010 – May 2011 was a 1-in-62,500 year event. The record 12-month minimum of 197 EF-1 and stronger tornadoes that occurred from May 2012 – April 2013 was a 1-in-3000 to 1-in-4000 year event. In Marsh’s words: “Anyway you look at it, the recent tornado “surplus” and the current tornado “drought” is extremely rare. The fact that we had both of them in the span of a few years is even more so!”

Our tornado drought may be at its end, as the latest forecasts from NOAA’s Storm Prediction Center call for an active severe weather pattern Saturday – Monday. The current forecast calls for just a “Slight Risk” on Saturday over the Northern Plains, but the threat will grow on Sunday and Monday as a powerful spring weather system gathers strength over the center of the country.

Figure 2. MODIS image of Tropical Cyclone Mahasen taken at 06:50 UTC Thursday May 16, 2013. Mahasen made landfall about two hour prior to this image as a tropical storm with 50 mph winds. Image credit: NASA.

Tropical Storm Mahasen hits BangladeshTropical Storm Mahasen hit the Bangladesh coast near 08 UTC Thursday, May 16 near a place called Feni north of Chittagong. Mahasen was a tropical storm with top winds of 50 mph at landfall. Satellite observations suggest that the storm was becoming much more organized just before landfall, and it is fortunate that the storm ran out of time to intensify when it did. Mahasen likely brought a storm surge of up to a meter (3.3 feet) to the coast of Bangladesh, but it is the storm’s rains that are causing the main problems. Satellite rainfall forecasts made at landfall show that Mahasen could dump up to 20 inches of rain along a swath through Bangladesh and into Northeastern India. These rains will be capable of causing destructive flooding, and ten deaths have already been reported in Bangladesh from the storm. At least eight people have been killed in Sri Lanka due to landslides triggered by Mahasen’s heavy rains, and a boat carrying refugees capsized on Monday, killing eight and leaving 50 missing.

First tropical storm of the year, Alvin, forms in the Eastern Pacific
The official start of hurricane season in the Eastern Pacific is Wednesday, May 15, and Mother Nature emphatically agreed, bringing us the first named storm of the year, Tropical Storm Alvin. With wind shear a moderate 10 – 20 knots and the storm currently struggling to hold itself together, it currently appears unlikely that we will see a Hurricane Alvin. The storm is moving west-northwest into the Central Pacific, and is not a threat to any land areas.